Messages that cannot be delivered for some reason are placed in the deferred queue. The reasons for the delivery failures are documented in a file in the deferred queue. This queue is scanned frequently to resend the message. If the message cannot be sent after the set number of delivery attempts, the message fails. The message is bounced back to the original sender. The default for the bounce queue lifetime is five days. You can change the default MTA value for bounce_queue_lifetime from the zmlocalconfig CLI.